Conventionally, as a system for desalinating seawater, there has been known a seawater desalination system in which seawater passes through a reverse-osmosis membrane-separation apparatus to remove salinity from the seawater. In the seawater desalination system, the intake seawater is processed to have certain water qualities by a pretreatment system, and the pretreated seawater is delivered into the reverse-osmosis membrane-separation apparatus under pressure by a high-pressure pump. Part of the high-pressure seawater in the reverse-osmosis membrane-separation apparatus passes through a reverse-osmosis membrane against the osmotic pressure and is desalinated, and fresh water (permeate or desalted water) is taken out from the reverse-osmosis membrane-separation apparatus. The remaining seawater is discharged in a concentrated state of a high salt content as a concentrated seawater (brine) from the reverse-osmosis membrane-separation apparatus. The largest operational cost in the seawater desalination system is energy cost, and it depends heavily on energy for pressurizing the pretreated seawater up to such a pressure to overcome the osmotic pressure, i.e. up to the reverse-osmosis pressure. That is, the operational cost of the seawater desalination system is greatly affected by pressurizing energy of the seawater by the high-pressure pump.
Specifically, more than half of the electric expenses as the highest cost in the seawater desalination system are consumed to operate the high-pressure pump for pressurizing the seawater. Therefore, pressure energy possessed by the high-pressure concentrated seawater (reject) with the high salt content which has been discharged from the reverse-osmosis membrane-separation apparatus is utilized for pressurizing part of the seawater. Therefore, as a means for utilizing the pressure energy of the concentrated seawater discharged from the reverse-osmosis membrane-separation apparatus to pressurize part of the seawater, there has been utilized an energy recovery chamber in which an interior of a cylinder is separated into two spaces by a piston arranged to be movable in the cylinder, a concentrated seawater port is provided in one of the two separated spaces to introduce and discharge the concentrated seawater, and a seawater port is provided in the other of the two separated spaces to introduce and discharge the seawater.
FIG. 15 is a schematic view showing a configuration example of a conventional seawater desalination system. As shown in FIG. 15, seawater pumped into the seawater desalination system by an intake pump (not shown) is processed to have certain water qualities by a pretreatment system for removing suspended matter or the like, and then the pretreated seawater is delivered via a seawater supply line 1 into a high-pressure pump 2 that is driven by a motor M. The seawater which has been pressurized by the high-pressure pump 2 is supplied via a discharge line 3 to a reverse-osmosis membrane-separation apparatus 4 having a reverse-osmosis membrane (RO membrane). The reverse-osmosis membrane-separation apparatus 4 separates the seawater into concentrated seawater with a high salt content and fresh water with a low salt content and obtains the fresh water from the seawater. At this time, the concentrated seawater with a high salt content is discharged from the reverse-osmosis membrane-separation apparatus 4, and the discharged concentrated seawater still has a high-pressure. A concentrated seawater line 5 for discharging the concentrated seawater from the reverse-osmosis membrane-separation apparatus 4 is connected via a control valve 6 to a concentrated seawater port P1 of an energy recovery chamber 10. A seawater supply line 1 for supplying the pretreated seawater having a low pressure is branched at an upstream side of the high-pressure pump 2 and is connected via a valve 7 to a seawater port P2 of the energy recovery chamber 10. The energy recovery chamber 10 has a piston 16 therein, and the piston 16 is arranged to be movable in the energy recovery chamber 10 while separating the interior of the energy recovery chamber 10 into two volume chambers.
The seawater pressurized by utilizing a pressure of the concentrated seawater in the energy recovery chamber 10 is supplied via the valve 7 to a booster pump 8. The control valve 6, the valve 7 and the energy recovery chamber 10 constitute an energy recovery apparatus 11. Then, the seawater is further pressurized by the booster pump 8 so that the seawater has the same pressure level as the discharge line 3 of the high-pressure pump 2, and the pressurized seawater merges via a valve 9 into the discharge line 3 of the high-pressure pump 2 and is then supplied to the reverse-osmosis membrane-separation apparatus 4.
FIG. 16 is a schematic view showing a configuration example of the conventional seawater desalination system comprising the two control valves 6, the two energy recovery chambers 10 and the two valves 7 which are the components of the energy recovery apparatus shown in FIG. 15. As shown in FIG. 16, since the energy recovery apparatus 11 has the two energy recovery chambers 10, 10, the energy recovery apparatus 11 is operated such that while the concentrated seawater is supplied to one of the two energy recovery chambers 10, 10, the concentrated seawater is discharged from the other of the energy recovery chambers. Therefore, since the high-pressure seawater can be discharged at all times (continuously) from the apparatus by alternating suction of the low-pressure seawater and discharge of the high-pressure seawater, the flow rate of the seawater supplied to the reverse-osmosis membrane separation apparatus 4 can be kept constant and the fresh water can be obtained at a constant flow rate from the reverse-osmosis membrane separation apparatus 4.
In the above-described conventional energy recovery chamber, the piston in the energy recovery chamber is brought into sliding contact with the inner wall of the chamber, and thus the sliding member of the piston is required to be periodically replaced due to wear of the sliding member. Further, the inner diameter of the long chamber is required to be machined with high accuracy so as to fit with the outer shape of the piston, and thus machining cost is very expensive.
Therefore, the applicants of the present invention have proposed an energy recovery chamber having no piston in Japanese-laid open patent publication No. 2012-232291 by employing the system for pressurizing the seawater directly with the concentrated seawater by introducing the seawater and the high-pressure concentrated seawater discharged from the reverse-osmosis membrane (RO membrane) into a cylindrical and elongated chamber, which is used as an energy exchange chamber.
FIG. 17 is a cross-sectional view showing an energy recovery chamber 10 having no piston. As shown in FIG. 17, the energy recovery chamber 10 comprises a long chamber body 11 having a cylindrical shape, and end plates 12 for closing both opening ends of the chamber body 11. A chamber CH is formed in the chamber body 11, and a concentrated seawater port P1 is formed in one of the end plates 12 and a seawater port P2 is formed in the other of the end plates 12. The concentrated seawater port P1 and the seawater port P2 are disposed on the central axis of the cylindrical chamber body 11. The inner diameter of the chamber CH is set to ϕD, and the inner diameter of the concentrated seawater port P1 and the seawater port P2 is set to ϕd.
The energy recovery chamber 10 is installed vertically. The chamber CH is disposed vertically in consideration of the effect of a difference in specific gravity between the concentrate seawater and the seawater, and the port P1 for the concentrated seawater having large specific gravity is disposed at a lower part of the chamber CH and the port P2 for the seawater having small specific gravity is disposed at an upper part of the chamber CH. Specifically, the long chamber body 11 having a cylindrical shape is disposed such that a longitudinal direction (axial direction) of the chamber is placed in a vertical direction. The concentrated seawater port P1 is provided at the lower part of the chamber CH so as to supply and discharge the concentrated seawater at the lower part of the chamber CH, and the seawater port P2 is provided at the upper part of the chamber CH so as to supply and discharge the seawater at the upper part of the chamber CH. The entire length of the chamber CH is L. In the chamber CH, a flow resistor 13 is disposed at a position spaced by a distance L1 in the axial direction from the concentrated seawater port P1, and a flow resistor 13 is disposed at a position spaced by a distance L1 in the axial direction from the seawater port P2. The flow resistor 13 comprises a single perforated plate.
In the energy recovery chamber 10 shown in FIG. 17, a fluid flows in from the respective ports P1, P2 having a small diameter, and the fluid flow having a large velocity distribution at a central part of the chamber is dispersed in a diametrical direction of the chamber CH by the flow resistor 13 and is thus regulated to form a uniform flow in the cross-section of the chamber. Therefore, two fluids are pushed and pulled in such a state that the interface between the seawater and the concentrated seawater is maintained horizontally, and thus the energy transmission is performed while maintaining the state in which the seawater and the concentrated seawater having different salt concentrations are less likely to be mixed in the chamber.
FIG. 18 is a cross-sectional view showing the energy recovery chamber 10 in which two perforated plates spaced by a predetermined distance are disposed near the respective ports as a flow resistor disposed near each port in FIG. 17. As shown in FIG. 18, in the chamber CH, a first perforated plate 14 is provided at a position spaced by a distance L1 in the axial direction from the concentrated seawater port P1, and a second perforated plate 15 is provided at a position spaced by a distance L2 in the axial direction from the first perforated plate 14. Similarly, a first perforated plate 14 is provided at a position spaced by a distance L1 in the axial direction from the seawater port P2, and a second perforated plate 15 is provided at a position spaced by a distance L2 in the axial direction from the first perforated plate 14. The two perforated plates 14 and 15 constitute a flow resistor 13.
Other structural elements of the energy recovery chamber 10 shown in FIG. 18 are the same as those in the energy recovery chamber 10 shown in FIG. 17.